Hello, GraceMacbook,
I see that you're not able to open up Audio MIDI Setup and I'm glad to help you with this.
The first thing that I notice is that you should have a handful of updates for OS X El Capitan. You mentioned being on 10.11.1 but the most recent version of El Capitan was 10.11.6. I do also want to note that, if your MacBook is compatible, there have been several major releases of OS X - now called macOS - since the release of El Capitan. The current version is macOS Mojave (Version 10.14) and it has quite a few really great features on top of the features introduced in the updates between El Capitan and Mojave. You can learn more about the free macOS Mojave update here: macOS Mojave
However, if you're not quite ready to upgrade, I would still recommend making sure that your current operating system is fully up to date and then trying Audio MIDI Setup again. You can follow these steps to update your software: How to update the software on your Mac
I hope this information helps.
Cheers!